Influence of an experimental fish farm on the spatio-temporal dynamic of a Mediterranean maërl algae community

This work concerns the monitoring of a Mediterranean maërl bed influenced by an experimental fish cage culture for eighteen months. The maërl bed showed an a priori stratified distribution with depth, which led to the selection of a stratified random sampling design that also took into account the gradient of influence by the fish cages. Maërl coverage and total biomass, macroalgae species richness and community structure as determined from semi-quantitative abundance data were assessed. The content of organic matter sediment was also considered. Sampling campaigns (performed by scuba divers) were carried out six months before the start of the culture, twice during the culture, and two months after the end of the experimental culture. Below the fish cages, the maërl community was almost completely buried and dead blackened calcareous algae were seen beneath a thin layer of sediment prior to the end of the study, and only a few specimens of Lithophyllum racemus, Flabellia petiolata, Gracilaria cylindrica and Meredithia microphylla remained alive. The influence of the experimental culture on the maërl algae community structure bed did not extend beyond the facilities, but a regression of the community was also observed close to the fish cages. Sensitivity to aquaculture activity, and the ecological benefits provided by maërl beds suggest that this community warrants further consideration when planning fish farm site selection and management.

► Spatio-temporal dynamic of a maërl bed influenced by an experimental cage fish farm is assessed.
► A stratified random sampling design was set also considering an impact gradient.
► Maërl community practically disappeared below the cages after only a few months of farming activity.
► Spatial extent of the impact is confined to the vicinity of the farming facilities.